[clang] [llvm] [clang][Modules][Serialization] Avoid serializing duplicate source location entries for shared input files (PR #209795)

[Chuanqi Xu via cfe-commits]

================
@@ -6789,9 +6839,39 @@ unsigned ASTWriter::getAdjustedNumCreatedFIDs(FileID FID) const {
   return AdjustedNumCreatedFIDs;
 }
 
+SourceLocation ASTWriter::getRedirectedLocation(SourceLocation Loc) const {
+  if (NonAffectingRedirectAdjustments.empty())
+    return SourceLocation();
+
+  SourceLocation::UIntTy Offset = Loc.getOffset();
+  if (PP->getSourceManager().isLoadedOffset(Offset))
+    return SourceLocation();
+
+  unsigned Idx = getNonAffectingRangeLowerBound(Offset);
+  if (Idx == NonAffectingRanges.size())
+    return SourceLocation();
+
+  // The search only rules out ranges ending before the offset, so check that
+  // the offset really is inside the one we landed on.
+  if (Offset < NonAffectingRanges[Idx].getBegin().getOffset())
+    return SourceLocation();
+
+  int64_t Adjustment = NonAffectingRedirectAdjustments[Idx];
+  if (!Adjustment)
+    return SourceLocation();
+  return SourceLocation::getFileLoc(static_cast<SourceLocation::UIntTy>(
+      static_cast<int64_t>(Offset) - Adjustment));
----------------
ChuanqiXu9 wrote:

I failed to understand this. What is Adjustment? And why do we need Offset  - Adjustment?

https://github.com/llvm/llvm-project/pull/209795